Strong gravitational lensing as a probe of galaxy evolution and cosmology

In this thesis, I explore how the environments of both galaxy and cluster-scale strong gravitational lenses affect studies of cosmology and the properties of the earliest galaxies.;Galaxy-scale lenses with measured time delays can be used to determine the Hubble constant, given an accurate lens model. However, perturbations from structures along the line of sight can introduce errors into the measurement. I use data from a survey towards known lenses in group environments to calculate the external shear in these systems, which is typically marginalized over in standard lens analyses. In three of six systems where I compare the independently-calculated environment shear to lens model shears, the quantities disagree at greater than 95% confidence. We explore possible sources of this disagreement. Using these data, I generate fiducial lines of sight and insert mock lenses with assumed input physical and cosmological parameters and find that those parameters can be recovered with ∼ 5--10% scatter when uncertainties in my characterization of the environment are applied. The lenses in groups have larger bias and scatter. I predict how well new time delay lenses from LSST will constrain H0 and find that an ensemble of 500 quad lenses will recover H0 with ∼ 2% bias with ∼ 0.3% precision.;On larger scales, galaxy cluster lenses can magnify the earliest galaxies into detectability. While past studies have focused on single massive clusters, I investigate the properties of lines of sight, or ``beams", containing multiple cluster-scale halos in projection. Even for beams of similar total mass, those with multiple halos have higher lensing cross sections on average. The optimal configurations for maximizing the cross section are also those that maximize faint z ∼ 10 detections. I present a new selection technique to identify beams in wide-area photometric surveys that contain high total masses and often multiple clusters in projection as traced by luminous red galaxies. I apply this technique to the Sloan Digital Sky Survey and present the 200 most promising beams. Several are confirmed spectroscopically to be among the highest mass beams known with some containing multiple clusters. These are among the best fields to search for faint high-redshift galaxies.